This invention relates to the basic arrangement of nuclear power plants; in particular, to the arrangement of Pressurized Water Reactor (PWR) plants.
Besides the reactor, typical PWR plants have steam generators located inside a containment structure as shown in FIG. 1. The steam generator is a heat exchanger between the "primary loop" radioactive reactor coolant and the non-radioactive "secondary loop". In a PWR, the primary loop is sufficiently pressurized to suppress steaming. Recirculation pumps are used to circulate the hot water between the steam generators and the reactor. In the secondary loop, the feedwater pumps provide the clean water to the steam generators. The clean water is evaporated in the steam generator to provide clean steam to drive the turbines. The steam turbines rotate the electric generator.
In contrast to PWR plants, traditional Boiling Water Reactor (BWR) Plants do not have steam generators. In a BWR plant, the reactor provides radioactive steam directly to the steam turbines as shown in FIG. 2. This arrangement saves the expense of the steam generators and avoids the heat transfer inefficiencies, caused by the steam generators. On the other hand, BWR plants are faced with the problem of having radioactive steam in the secondary loop, necessitating shielding and restricting maintenance during operation.
Experience has shown that over time cracks develop in the thin-walled PWR steam generator tubes, causing radioactive steam leakage to the clean secondary loop. When significant radiation is detected in the secondary loop, the plant is shut down while the leaking tubes are plugged. After about ten percent of the tubes are plugged, or after frequent shut-downs, the steam generator is replaced or re-tubed. This operation results in a several-month-long down-time and is the most frequent problem encountered in present PWR plants. The expense of shutting down for months to replace the steam generators is so significant that it has led to the premature decommissioning of several plants, e.g. Trojan Generating Station in Portland, Oreg. Any improvement in steam generator life extension or ease of replacement is desirable. Better water quality, new tube materials, better manufacturing and quality procedures are the long touted panaceas for tube life extension. However, these solutions have resulted in only limited success. Consequently, plants are frequently shut down for extended periods of time for steam generator repair and replacement.